Glomerular permeability will be measured with polydisperse polyvinyl pyrrolidone I125 (PVP). Using column chromatography PVP will be separated according to molecular size and the clearance of different moeities of PVP will be calculated. Simultaneously clearances of inulin and PAH as well as filtration fraction will be measured and under same protocols renal tissue obtained by renal biopsy will be studied by light immunofluorescent and electron microscopy. Studies will be undertaken in rabbits with experimental glomerulonephritis and the influence of drugs (e.g. steroids, immuran or cytoxan) on the various parameters studied will be determined. Initially glomerulonephritis will be induced using anti-glomerular-basement membrane antibody, subsequent studies will be performed in immune-complex glomerulonephritis. Human renal disease will be studied, serial investigations being performed in children with either acute or chronic glomerulonephritis or idiopathic nephrotic syndrome. An attempt will be made to correlate renal pathology with abnormal patterns of glomerular permeability, to determine whether prognosis may be predicted from glomerular permeability and to determine whether therapy with the drugs enumerated above will modify glomerular permeability.